The invention relates to the field of communications and communication systems, more particularly, to a code division multiple access (CDMA) communication system.
Orthogonal transmit diversity (OTD) is a feature of a CDMA communication system implemented for improving performance of a communication link between a base station and a mobile station. The commonly known IS-2000 standard provides a detailed description for implementation and use of an OTD feature in a CDMA communication system. A copy of the IS-2000 standard may be obtained by contacting Telecommunications Industry Association, 2500 Wilson Blvd., Suite 300, Arlington, Va. 22201 USA, or visiting a world web site on the internet located at http://www.tiaonline.org/, which incorporated by reference herein. The OTD feature is applicable for both single carrier CDMA system and multi-carriers (MC) CDMA system.
Generally speaking, information data in a CDMA system is coded according to a coding rate. For example, coding rates of 1/3, 1/2 and 1/4 are commonly used in CDMA systems operating according to IS-2000 standards; other coding rates are also available. For example, when the information data is coded according to a 1/3 coding rate, the encoder produces 3 data symbols for every input information data bit, similarly for other encoding rates. One ordinary skilled in the art may appreciate that data symbol and data bit are interchangeable terms. After the information data has been encoded, the encoded symbols are transmitted to a receiving unit for decoding and recovery of the information data bits. The transmission of data normally takes place over a channel with varying propagation characteristics. As a result, if the encoded symbols are transmitted sequentially from a transmitting source such that all or most of the encoded symbols experience the same or similar propagation characteristics in the transmission channel, the information data may not be decoded or recovered at the receiver due to severe degradation in the received signal energy of all or most of the encoded symbols.
The OTD provides a method for transmission of encoded symbols such that, as much as possible, each encoded data symbol of an information data bit experiences a propagation characteristics independent of other encoded symbols of the same information data bit. As such, the possibility of degrading all or most of the encoded data symbols of the same information data bit due to a varying propagation channel characteristics is spread and minimized. A system implementing the OTD feature transmits encoded data symbols of an information data bit from at least two transmit antennas, such that one encoded symbol is transmitted from one antenna and another encoded symbol from another antenna. The encoded symbols transmitted from each antenna are spread using an orthogonal Walsh code different than the Walsh code used for the encoded symbols transmitted from the other antenna. A mobile station, or a remote receiving unit, receiving both signals obtains a reception diversity improvement by allowing the decoder to process a group of symbols that have faded independently. In order to ensure that coded symbols from the same information data bit fade independently in transmission, use of two parallel interleavers in the transmitter may be required according to the description provided in the earlier versions of IS-2000 standard and related literatures. Referring to FIG. 1, a CDMA communication system transmitter block diagram 100 is shown. Raw information data bits are added with CRC bits and some other tail data bits in a block 101 to produce information data bits 102. Information data bits 102 are fed to a channel encoder 103 for encoding according to an encoding rate to produce encoded symbols 104. In case of implementing an OTD feature, the encoded symbols 104 are fed to an OTD interleaver 105; otherwise, the encoded symbols 104 are fed to a single interleaver before further processing of the transmit signal. After interleaving, interleaved symbols 106 are produced. Interleaved symbols 106 pass through a transmit signal processing block 107 which may include long code masking, power control operation and power control puncturing operations; such operations are well known in the art and more specifically explained in the IS-2000 standard. Block 107 produces data symbols 108 which pass through a data splitter 109 to produce two streams of data symbols 110 and 111. The data symbols 110 and 111 are modulated independently in QPSK modulators 112 and 113 and up-converted to an appropriate carrier frequency before transmission from antennas 114 and 115. On a receiving end both signals transmitted from antennas 114 and 115 are demodulated, decoded and combined to recover the information data bits.
The OTD interleaver 105 requires at least two data block interleavers 120 and 121. The information data bits 102 are normally transmitted in a block of data format, as such, the encoded symbols are also in a block of encoded symbols format. Normally, a block of information data bits is set to occupy a predefined time frame. The number of information data bits in a time frame varies depending on the information data bit rate. Time frames of 5, 10, 20, 40 and 80 mSec. are possible. The information data rate also may vary from 9.6 kbps up to 460.8 kbps. Calculating the number of information data bits or encoded symbols in a time frame is well known by one ordinary skilled in the art. Similarly, the number of encoded symbols depends on the encoding rate and the number of information data bits in a time frame. For example, one block of encoded symbols 104 in a 20 mSec time frame may include 384 encoded symbols for an information data bit rate 9.6 Kbps in a 20 mSec time frame using an encoding rate of 1/2.
A block of encoded symbols 104 are de-muxed in a de-mux block 122 to produce two halves, blocks of symbols 123 and 124 which have the same time frame and equal to the time frame of the block of encoded symbols 104. The number of symbols in each block of symbols 123 and 124 is equal to one half of the encoded symbols in the block of encoded symbols 104. Two data block interleavers 120 and 121 according to a prior art provide the interleaving functions for interleaving each block of symbols 123 and 124. If the number of encoded symbols in the block of encoded symbols 104 is equal to "N", the number of symbols in each block 123 and 124 is equal to "N/2". As a result, the interleavers 120 and 121 perform interleaving functions for "N/2" symbols. The operation of interleaving is according to a known and predetermined algorithm. Such algorithm often may be governed by a mathematical relationship mapping the input to output symbols. The interleavers 120 and 121 then output interleaved symbols 125 and 126 of block of encoded symbols 123 and 124. The OTD interleaver 105 may additionally for improving performance may include a circular shift operation 127 shifting data symbols by a predetermined number. The circular shift function 127 shifts the data symbols of, in this case, the interleaved symbols 126 to produce data symbols 128. Data symbols 128 and 125 are muxed in a mux-ing block 129 to produce interleaved symbols 106.
Referring to FIG. 2, a case of multi-carrier (MC) CDMA communication system, the OTD MC interleaver 201 may be used. Interleaver 201 inputs encoded symbols 104 and outputs interleaved symbols 106 according to its internal OTD operations. A block of input encoded symbols is de-muxed into three blocks 202, 203 and 204 in a demux-ing block 205 for multi-carrier CDMA system with three CDMA carriers. Each block of symbols 202, 203 and 204 pass through an interleaver block respectively shown as interleaver blocks 206, 207 and 208 to produce interleaved blocks of symbols 209, 210 and 211 respectively. Interleaved symbols 210 and 208 pass through circular shift operations 212 and 213 to produce symbols 214 and 215. Symbols 209, 214 and 215 are muxed in a mux-ing operation 216 to produce interleaved symbols 106. In case of OTD-MC interleaver 201, the data splitter 109 splits the data block into three blocks of data for transmission from up to three transmit antennas. The modulation and frequency up-conversion of transmission may be performed independently for three carrier frequencies. Each carrier may be transmitted from a respective antenna, or two carries from one antenna and the third one from a second antenna.
As shown in FIGS. 1 and 2, the prior art requires at least two block interleavers, and in case of MC-OTD, at least three block interleavers. The operations within such block interleavers are intensive and are often governed by one or more mathematical relationships to map the data symbols of an input block of data symbols to data symbols of an output block of data symbols, in addition to other operations such as muxing, de-muxing and circular shifts operations. As such, operations relating to multiple block interleavers take extensive hardware and software resources.
Therefore, there is a need for a method and apparatus for providing OTD feature in a CDMA communication system without a requirement of multiple block interleavers.